Optimized finned absorber geometries for solar air heating collectors

Solar air heaters are limited in their thermal performance due to the low density, the small volumetric heat capacity and the small heat conductivity of air. For high solar gains, an efficient thermal coupling between absorber and fluid is required, while the electrical power for the fan operation ought to be as small as possible. As heat transfer augmentation usually increases friction losses, an optimum fin geometry has to be found. For a solar ventilation air preheater, mounted on a south facing facade in Wurzburg, Germany, optimized geometries have been derived. For a specific air mass flow rate of 70 kg/(m(2) h) the air gap For smooth absorbers without fins should be about 7 to 8 mm to get a maximized yearly net energy output of about 680 MJ/m(2) during the heating season from October through April. Finned absorbers perform much better. Continuous aluminum-fins, 0.1 mm thick and spaced about 6 mm apart in a 30-mm-wide air gap yield about 900 MJ/m(2) yearly net output. Offset strip fins do not show an improved performance compared to optimally spaced continuous fins, due to the larger electrical power for this geometry. However, offset strip fins yield high net energy gains for large fin spacings. (C) 2000 Elsevier Science Ltd. All rights reserved.